The goal of this research is to understand the mechanisms that regulate tonic force maintenance with a very low energy usage and slow crossbridge cycling rate in smooth muscle. These are characteristics of a "catch" muscle, the anterior byssus retractor (ABRM) muscle of the mussel Mytilus edulis, the prototype of the "latch" state of vertebrate smooth muscle. We showed that the phosphorylation state of the mini-titin, twitchin, located on the thick filament, is a major site of regulation of force production and maintenance. Catch force is associated with the dephosphorylation of twitchin, and release of catch, or relaxation, is associated with its protein kinase A mediated phosphorylation. We have also characterized distinct mechanical states and their regulation by twitchin phosphorylation, and determined the sequence of Mytilus twitchin. The ABRM is an ideal model because of the ease (a) by which distinct mechanical states (calcium-driven cycling, non-cycling or catch crossbridges, relaxation) can be produced in intact and permeabilized muscles, and (b) of controlling the regulatory mechanisms, i.e., the state of twitchin phosphorylation and calcium. Conserved portions of proteins such as titin, projectin, C-protein and caldesmon, implicated in the regulatory processes of contraction are similar to portions of twitchin, suggesting its role in regulatory processes in general. The SPECIFIC AIMS are: (1) Determination of the role of twitchin phosphorylation-mediated changes in ADP binding to and release from myosin in controlling the mechanical states of catch muscle. (2) Determination of the mechanism by which twitchin regulates catch through measurement of the effects of recombinant portions of the molecule on mechanical parameters in permeabilized muscle.(3) Determination of the thick and thin filament binding characteristics of twitchin and recombinant portions of twitchin. (4) Determination of the extent of involvement of myosin crossbridges in catch force maintenance by measurements of changes in orientation of fluorescent probes attached to the regulatory light chain.